Mind you, the black coloring is there for a reason, and you might not know that since you've not read it since impel Down.
Luffy using one of the generic, non-devilfruit "Haki" powers, which hardens his body to be as strong as steel and allows him to hit even some of the super overpowered devilfruit users that can transform their whole bodies into some element. It also turns the skin black. Luffy uses it a lot post-timeskip, and lo and behold, his Gear 4th uses it as well.
Okay, here's the thing. You can argue art style, but art quality in One Piece manga is solid at the very least, especially when compared to Naruto and Bleach. These might not be the most difficult competition, but remember, "Big Three", all of them wrote weekly chapters at the same time for years.
In any given page, Oda does not take shortcuts or skip details even when the panels get really tiny or really busy (sometimes to his detriment even). He'll do the shading with appropriate hatching, will draw the patterns on clothes, give proper structure and texture to all the enemies in the picture, keep the designs consistent, and won't just draw a pure white background or speedline panels nearly as often as the other two.
Comparing the art of the last 100 chapters of the Big Three shows the difference very clearly. Kishimoto abuses speed line panels and character proportions will change until you might think it's a different artist. And I do not know whether it's due to the shortcuts or in spite of them, a lot of the fight scenes in Naruto are just incomprehensible.
Kubo has everyone fighting in a featureless desert, gives everyone simple black&white costumes because those are easy to draw and color. On top of that, he'll draw constant close-up shots of people just standing and talking to each other on said desert.
